Problem: $ B = \left[\begin{array}{rrr}1 & 2 & 1 \\ 2 & 1 & 1 \\ 0 & 0 & 1\end{array}\right]$ What is $ B^{-1}$ ?
Explanation: $ = \left[\begin{array}{rrr}-\frac{1}{3} & \frac{2}{3} & -\frac{1}{3} \\ \frac{2}{3} & -\frac{1}{3} & -\frac{1}{3} \\ 0 & 0 & 1\end{array}\right]$